Read Gum-Bichromate Printing text version

Potassium Bichromate Water Gum (water soluble) Colours Notes: Potassium bichromate is now known as potassium dichromate. The gum must be a water-based type, e.g. gum arabic, Gloy or similar. Colours must be water-based, e.g. water colours, gouache or poster colours. 5 gm 100 ml

Method

1. 2. 3. 4. 5. 6. 7. Dissolve 5 gm of potassium bichromate in 100 ml water. Mix 5ml of bichromate solution with 5 ml gum. Add the colour. As a rough guide squeeze out about ¼&quot; of colour from the tube. Mix thoroughly. Brush the liquid on to the paper and then dry it. Expose the paper in contact with the negative either in a UV box or in daylight. Rinse the paper in warm (40 oC) water. (It may require some gentle brushing to remove the unhardened gum.) Dry the paper.

8.

Notes Papers Choose a paper with some surface texture and which is well sized. Most good quality watercolour papers will fit the bill, e.g. Fabriano, Bockingford, Arches, Rives or similar. However,

CFW/2009

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Gum Bichromate Printing

the gum-bichromate process is quite robust and other papers may well suit, provided they will withstand washing in warm water. Colours Water colour paints give the most transparent and delicate prints but the colour is so thin that several exposures may be necessary to build up sufficient density, in which case it will be necessary to ensure that the neg and print are exactly registered for each exposure. To achieve this, it is necessary to punch the neg and print using either a registration punch or a 3 or 4 hole paper punch and registration pins. Gouache is more dense and poster colours are the most dense. You will need to experiment, possibly with combinations of each to achieve the effect you want. Dry powder pigments can also be used provided that they will disperse in water.

Applying multiple colours Further layers of colour can be applied to build up a multicolour print. Before applying another layer, the paper must be sized with gum and water to protect the first layer. The second colour is then applied on top of this. For each colour, a mask will need to be cut from thick black paper to ensure that only those areas are exposed which are to be in that colour.

Health and Safety

Potassium dichromate is a vesicant (causes blisters) and, being a compound of chromium, is potentially carcinogenic. However, as long as it is handled with reasonable caution it is no more hazardous than many household chemicals such as oven cleaners and drain cleaners.